1. Field of the Invention
The present invention relates to a motor for use in an information recording device, such as an FDD, for example, which accesses information by rotating a disk-shaped recording medium at a constant speed, and in particular, it relates to a spindle motor which drives a recording medium directly.
2. Description of the Related Art
An FDD (Floppy Disk Drive) is used for writing and reading information to and from a disk-shaped information recording medium, known as a floppy disk, used in word processors and personal computers. In the prior art, generally, a brushless DC motor has been employed as the spindle motor for driving rotation of a floppy disk in an FDD. Brushless DC motors present no problems whatsoever in terms of performance or reliability, but since they require rotor position detectors and a speed detector, the motors themselves are unavoidably expensive. Moreover, feedback control of the rotational speed is required in the drive, in order to ensure rotational accuracy, and hence the circuit composition becomes large in size, and such systems have been unable to meet recent demands for cost reduction. In particular, the spindle motor represents a large cost element in the device, and lowering the cost of the spindle motor has become an important objective.
On the other hand, due to the progress of digital electronics technology in recent years, it is possible to drive rotation using an inexpensive stepping motor (a two-phase claw pole motor is especially effective) as a synchronous motor which is synchronized to a clock by means of a simple drive circuit. Furthermore, since a digital drive circuit can be constructed without the inclusion of rotor position detectors or a speed detector in the motor, it has a merit in that the system as a whole can be constructed very inexpensively, and employment thereof is under investigation. However, there is large variation in the rotation of stepping motors due to the step movement of their basic operation, and in particular, in current circumstances, they cannot be employed as spindle motors for information recording devices where high rotational accuracy is demanded.
Furthermore, the use of circuit technology, such as micro-stepping or the like, in order to improve rotational accuracy, is also under study, but this has a drawback in that the cost of the drive circuit becomes extremely high.